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Photocopying is a process which makes
paper copies of
documents and other visual images quickly and cheaply. Most current photocopiers
use a technology called xerography,
a dry process using heat. (Copiers can also use other output technologies such
as
ink jet, but xerography is standard for office copying.)

Photocopying is widely used in business, education, and government. There
have been many predictions that photocopiers will eventually become obsolete as
information workers continue to increase their digital document creation and
distribution, and rely less on distributing actual pieces of paper. However,
photocopiers are undeniably more convenient than computers for the very common
task of creating a copy of a piece of pa

How a photocopier works (using xerography)

Schematic overview of the xerographic photocopying process (step 1-4)

Charging: The surface of a cylindrical drum is given an
electrostatic charge by either a high voltage wire called a corona wire or a
charge roller. The drum is coated with a photoconductive material, such as
selenium.
A
photoconductor is a
semiconductor that becomes conductive when exposed to light.

Exposure: A bright lamp is shined onto the original document, and
the white areas of the original document reflect the light onto the surface of
the photoconductive drum. The areas of the drum that are exposed to light
(those areas that correspond to white areas of the original document) become
conductive and therefore discharge to ground. The area of the drum not exposed
to light (those areas that correspond to black portions of the original
document) remain negatively charged. The result is a latent electrical image
on the surface of the drum.

Developing: The toner is positively charged. When it is applied to
the drum to develop the image, it is attracted and sticks to the areas that
are negativly charged (black areas), just as paper sticks to a toy balloon
with a static charge.

Transfer: The resulting toner image on the surface of the drum is
transferred from the drum onto a piece of paper with a higher negative charge
than the drum.

Fusing: The toner is melted and bonded to the paper by high-heat
and high-pressure rollers.

Cleaning: The drum is wiped clean with a rubber blade and
completely discharged by light, before beginning the process again.

This example is of a negatively charged drum and paper, and positively
charged toner. Some copiers employ the opposite: a positively charged drum and
paper, and negatively charged toner.

Invention

In 1937Bulgarianphysicist
Georgi Nadjakov found that when placed into electric field and exposed to
light, some dielectrics acquire permanent electric polarization at the exposed
areas.[1]
That polarization persists in the dark and is destroyed in light.
Chester Carlson, the inventor of photocopying, was originally a
patent attorney and part time researcher and inventor. His job at the patent
office in New
York required him to make a large number of copies of important papers.
Carlson, who was
arthritic,
found this a painful and tedious process. This prompted him to conduct
experiments with
photoconductivity. Carlson experimented with "electrophotography"
in his kitchen and in
1938, applied for a patent for the process. He made the first "photocopy"
using a zinc plate
covered with sulfur.
The words "10-22-38 Astoria" were written on a
microscope
slide, which was placed on top of more sulfur and under a bright light. After
the slide was removed, a mirror image of the words remained. Carlson tried to
sell his invention to some companies, but because the process was still
underdeveloped he failed. At the time multiple copies were made using carbon
paper or duplicating machines, and people did not feel the need for an
electronic machine. Between
1939 and
1944, Carlson was
turned down by over 20 companies, including
IBM and
GE, neither of which believed there was a significant market for copiers.

In 1944 the
Battelle Memorial Institute, a non-profit organization in
Columbus, Ohio, contracted with Carlson to refine his new process. Over the
next five years, the institute conducted experiments to improve the process of
electrophotography. In
1947Haloid
(a small New York based organisation manufacturing and selling photographic
paper at that time) approached Battelle to obtain a license to develop and
market a copying machine based on this technology.

Haloid felt that the word "electrophotography" was too complicated and did
not have good recall value. After consulting a professor of classical language
at
Ohio State University, Haloid and Carlson changed the name of the process to
"Xerography",
derived from
Greek words which meant "dry writing". Haloid decided to call the new copier
machines "Xerox" and in
1948, the word Xerox was trademarked.

In the early 1950s, RCA (Radio Corporation of America) introduced a variation
on the process called
Electrofax
where images are formed directly on specially coated paper and rendered with a
toner dispersed in a liquid.

Use

In 1949, the
Xerox introduced the first xerographic copier called Model A. Xerox became so
successful that photocopying came to be popularly known as "Xeroxing", a
situation that Xerox has very actively fought in order to prevent "xerox" from
becoming a
genericized trademark. "Xerox" has been found in some dictionaries as the
synonym of photocopying, leading to letters and ads from the Xerox corporation
asking that the entries be modified, and that people not use the term "Xerox" in
this way. However, this is mainly only true for
North
America - for example, in the
United Kingdom the term "photocopying" is far more common than "Xeroxing",
probably due to photocopiers from
Japanese and
European
manufacturers being far more commonly available than Xerox machines when
photocopying started becoming popular. "Photostat" is an outdated term for a
photocopy, which some in the United Kingdom still use. Some languages use hybrid
terms, such as widely used in
Polish term kserokopia ("xerocopy"), even despite relatively low
percentage of the copying machines available being branded Xerox.

Advances in technology developed the process of electrostatic copying
technology where a high contrast electrostatic image copy is created on a drum
and then a fusible plastic powder (called
toner) is
transferred to regular paper, heated and then fused into the paper similar to
the technology used in
laser
printers.

Prior to the widespread adoption of xerographic copiers, photo-direct copies
produced by machines such as Kodak's Verifax were used. A primary obstacle
associated with the pre-xerographic copying technologies was the high cost of
supplies: a Verifax print required supplies costing USD $0.15 in 1969, when a
Xerox print could be made for USD $0.03 including paper and labour. At that
period, Thermofax photocopying machines in libraries would make letter-sized
copies for $0.25 or more, when the minimum wage for a US worker was USD $1.65.

Xerographic copier manufacturers took advantage of the high perceived-value
situation of the 1960s and early 1970s and marketed paper that was "specially
designed" for xerographic output. By the end of the 1970s paper producers had
made xerographic runnability one of the requirements for most of their office
paper brands.

Digital technology

In recent years, all new photocopiers have adopted
digital
technology, replacing the older
analog technology. With digital copying, the copier effectively consists of
an integrated
scanner and
laser
printer. This design has several advantages, such as automatic image quality
enhancement and the ability to "build jobs" or scan page images independently of
the process of printing them. Some digital copiers can function as high-speed
scanners; such models typically have the ability to send documents via email or
make them available on a local area network.

The greatest advantage of a digital copier is "automatic digital
collation."
When copying a set of twenty pages twenty times, for example, a digital copier
scans each page only once, then uses the stored information to produce twenty
sets. In an analog copier, either each page is scanned twenty times (a total of
400 scans), making one set at a time, or twenty separate output trays are used
for the twenty sets.

Low-end copiers also use
digital
technology, but they tend to consist of a standard PC scanner coupled to an
inkjet or low-end laser printer, both of which are far slower than their
counterparts in high-end copiers. However, low-end scanner-inkjets can provide
color copying for a far lower cost than a traditional color copier. The cost of
electronics is such that combined scanner-printers sometimes have built-in fax
machines. (See
Multifunction printer.)

Color photocopiers

Colored toner became available in the
1950s, though
full color copiers were not commercially available until
3M released the
Color-in-Color copier in
1968, which used a
dye sublimation process rather than the normal electrostatic technology. The
first electrostatic color copier was released by
Canon
in 1973.

Color photocopying is a concern to
governments
since it makes
counterfeitingcurrency
much simpler. Some countries have introduced anti-counterfeiting technologies
into their currency specifically to make it harder to use a color photocopier to
counterfeit. These technologies include watermarks, microprinting,
holograms,
tiny security strips made of plastic or other material, and ink that appears to
change color as the currency is viewed at an angle. Some photocopying machines
contain special
software that will prevent the copying of currency that contains a
special pattern.

Copyright issues

Photocopying material which is subject to
copyright
(such as books or scientific papers) is subject to restrictions in most
countries. It is common practice, especially by students, as the cost of
purchasing a book for the sake of one article or a few pages may be excessive.
The principle of
fair use
(in the United States) or
fair
dealing (in other
Berne Convention countries) allows this type of copying for research
purposes.

In some countries, such as
Canada, some
universities pay royalties from each photocopy made at university copy
machines and copy centers to
copyright collectives out of the revenues from the photocopying and these
collectives distribute these funds to various scholarly publishers. In the
United States, photocopied compilations of articles, handouts, graphics, and
other information called readers are often required texts for college
classes. Either the instructor or the copy center is responsible for clearing
copyright for every article in the reader, and attribution information is
included in the front of the reader.

Health Issues

Ultraviolet exposure does remain a concern. In the early days of
photocopiers, the sensitizing light source was filtered green to match the
optimal sensitivity of the photoconductive surface. This filtering conveniently
removed all ultraviolet
[2]. Today a variety of light sources may be used. As
glass transmits
ultraviolet rays between 325 and 400 nanometers, copiers with
ultraviolet-producing lights such as fluorescent, tungsten halogen or xenon
flash will expose documents to some ultraviolet
[3].

Forensic identification

Similar to
forensic identification of
typewriters,
computer printers and copiers can be traced by imperfections in their
output. The mechanical tolerances of the toner and paper feed mechanisms cause
banding,
which contain information about the individual device's mechanical properties.
It is usually possible to identify the manufacturer and brand, but in some cases
the individual printer can be identified from a set of known ones by comparing
their outputs.
[4]
[5]

In 2005 some high quality color printers and copiers were demonstrated to
steganographically embed their identification code into the printed pages,
as fine and almost invisible patterns of yellow dots. This reportedly had been
done by top-of-the-line copiers for several years. The sources identify
Xerox and
Canon
as firms engaging in this practice.
[6]
[7] The US government has been reported as asking these companies to
implement such a tracking scheme so that
counterfeiting could be traced.

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